This invention relates to ambient cure compositions based on the base-activated Carbon Michael reaction between active methylene groups and active alkene groups. More particularly, the invention is directed toward use of tertiary amines and epoxides to activate the Carbon Michael reaction. In more specific aspects, the invention is directed toward classes of active methylene groups, active alkene groups, tertiary amines and epoxides that provide low cost, color and hazard, in two-pack coatings with good pot life, cure speed, gloss and durability on exposure to high humidity and ultraviolet light. Two-pack aliphatic urethane coatings represent the best current technology and provide targets for pot life, cure speed, gloss and durability, but alternatives to urethanes are needed with advantages in economy, safety, and ease of handling, especially for coatings with low levels of volatile solvent.
Heckles, U.S. Pat. Nos. 4,217,396, 4,217,439, 4,218,515 and 4,229,505 teach crosslinked polymers from polyfunctional acrylates and difunctional acetoacetates, diacetoacetamides, ureadiacetoacetamides and cyanoacetates with diacetoacetoamides and ureadiacetoacetamides. The crosslinking is activated by strongly basic catalysts such as sodium methoxide, sodium metal, sodium ethylate, and benzyl-trimethyl ammonium methoxide.
Bartman et al, U.S. Pat. No. 4,408,018 also teaches the use of base catalysts with sufficient activity to activate Michael cure reactions, such as potassium hydroxide, tetrabutyl ammonium hydroxide, potassium amylate, sodium methoxide, potassium ethoxide and other alkali metal derivatives of alcohol, and quaternary ammonium bases. However, Bartman et al note that amines are generally not sufficiently strong to catalyze the Michael reaction between acrylic polymers having pendant acetoacetate groups and multifunctional acrylic esters.
Brindopke et al, Australian Patent No. 8540807-A teaches that strong bases such as alkali metal hydroxides or alcoholates cause yellowing and cloudiness with the acrylic polymers having pendant acetoacetate groups. They specify other active methylene compounds, with activation by diazabicyclooctane (triethylenediamine, DABCO); halides of quaternary ammonium compounds, especially fluorides; organic phosphonium salts; amidines, such as tetramethylguanidine, diazabicycloundecene, and diazabicyclononene; phosphanes; alkali metal alcoholates; and quaternary ammonium compounds, such as alkylammonium, arylammonium and/or benzylammonium hydroxides or carbonates. Brindopke et al also teach that these catalysts or catalyst mixtures can be used in the presence of tertiary aliphatic amines which in themselves are not active at room temperature. Brindopke et al teach use of a broad range of compounds having at least two active alkene groups, excluding only those taught by Bartman et al. They specifically include derivatives of cinnamic acid, crotonic acid, citraconic acid or anhydride, mesaconic acid, fumaric acid, dehydrolevulinic acid or sorbic acid, but prefer acrylic acid, methacrylic acid and/or maleic acid or anhydride.
There are problems associated with use of all of the above catalysts for cure of those Carbon Michael-Reactive components which are preferred on the basis of cost, hazard to users and reactivity at ambient temperature.
With the components taught by Bartman et al, combining good cure with pot life is problematic: at the level of base catalyst required to give good ultimate cure, the pot life of the composition is too short. By use of quaternary ammonium salts of volatile acids, this problem can be overcome, but then combination of early cure and gloss becomes problematic: with salts of carbonic acid the surface cure is too fast at levels of catalyst giving good ultimate cure, resulting in low gloss from shrinkage due to loss of solvent after the surface has crosslinked, while with salts of less volatile acids the cure is slower than desired.
Also, as mentioned by Bartman et al, combination of the required components in two stable packages is problematic: bases strong enough to activate the cure typically lose activity on extended aging when combined with either compositions having pendant acetoacetate groups or compositions having other than the most resistant classes of ester bonds. In particular, activity is lost on storage of the active base catalyst with acrylic polymers containing acetoacetoxyethyl methacrylate (AAEM) and the polyacrylate crosslinkers taught in U.S. Pat. No. 4,408,018. This severely limits the packaging options: the systems either have three packages, one for each component, or one large and one very small package, the small package containing the catalyst. It is desired to have only two packages of the same order of magnitude in size.
Similar problems exist when the Carbon Michael-reactive methylene groups are esters of acetoacetic acid other than acrylic polymers containing acetoacetoxyethyl methacrylate, or when the Carbon Michael-reactive methylene groups are esters of malonic or cyanoacetic acid.
Similar problems exist when the Carbon Michael-reactive alkene component is a polyester containing fumarate and maleate moieties. Use of such polyesters as alkene component is highly desirable because they are less irritating than most multifunctional acrylates, and also because of potential for low cost. However, in addition to these pot-life/cure, cure rate/gloss, and packaging problems associated with the activators taught previously, their utility is problematic for another reason: such polyesters give weaker compositions than desired when the Carbon Michael-reactive methylene component is low in functionality/molecule. When the Carbon Michael-reactive methylene component is high enough in molecular weight and active methylene content per molecule for good strength and hardness, as with acrylic polymers containing pendant acetoacetate moieties, achieving good gloss and durability is problematic, probably because of incompatibility or phase-separation between the relatively high molecular weight alkene and methylene components.
Still another problem with polyesters containing fumarate and maleate moieties is their susceptibility to hydrolysis. It is known to minimize this by use of 2,2-alkylsubstituted 1,3-propanediols as glycols, however then polyesters tend to be readily crystallizable with neopentyl glycol and expensive with less readily available 2,2-alkylsubstituted 1,3-propane diols.
A solution to the above problems is needed to provide alternatives to urethanes for ambient cure, especially for applications requiring exterior durability.